Monocrystalline silicon essential for the manufacture of semiconductor devices and the like is grown by a CZ method or an FZ method, and a polycrystalline silicon rod or a polycrystalline silicon ingot is used as a raw material at that time. Such polycrystalline silicon materials are manufactured by a Siemens method in most cases (see, for example, Patent Literature 1). The Siemens method is a method for bringing a silane raw material gas, such as trichlorosilane or monosilane, into contact with a heated silicon core wire to vapor phase-grow (deposit) polycrystalline silicon on the surfaces of the silicon core wire by a CVD (Chemical Vapor Deposition) method.
For example, a polycrystalline silicon ingot is charged into a quartz crucible when monocrystalline silicon is grown by a CZ method, and a seed crystal is immersed in a silicon melt into which this ingot has been heat-melted to annihilate dislocation lines (make the seed silicon dislocation-free). Thereafter, the seed silicon is gradually increased in diameter until a predetermined diameter is reached, and crystal pull-up is performed. If any unmelted polycrystalline silicon remains in the silicon melt at this time, pieces of this unmelted polycrystalline silicon drift in the vicinity of a solid-liquid interface due to convection. Thus, these polycrystalline pieces may be a cause for the induction of dislocation generation and the loss of crystal lines.
In addition, in Patent Literature 2, there is pointed out a problem in which acicular crystal may deposit within a polycrystalline silicon rod (polycrystalline silicon bar) in a process for manufacturing the rod by a Siemens method and, if monocrystalline silicon is grown by an FZ method using such a polycrystalline silicon rod, individual crystallites do not melt uniformly according to their sizes due to the abovementioned non-uniform microstructure, and unmelted crystallites pass through a melt zone as solid particles into a monocrystalline rod as unmelted particles to be incorporated into coagulated surfaces of a monocrystal, thus causing defect formation.
For this problem, Patent Literature 2 proposes a technique of grinding or polishing surfaces of a sample cut out perpendicularly to the long-axis direction of the polycrystalline silicon rod, measuring the size and area ratio of acicular crystal by enhancing the contrast to the extent of being able to visually recognize the microcrystallites of a composition even under an optical microscope after etching, and determining the acceptance/rejection of the rod as a raw material for producing FZ monocrystalline silicon on the basis of the results of measurement.